1. Field
The present disclosure relates generally to electric motors, such as brushless direct current (DC) motors, and to the control of such electric motors. More particularly, the present disclosure relates to controlling the current in the windings of a brushless DC motor using impulse width modulation.
2. Background
Aircraft may employ various electronic devices and systems to perform various functions on the aircraft. For example, without limitation, electric motors on an aircraft may be used to move flight control surfaces, to raise and lower landing gear, and to perform other functions on the aircraft. Power for the electric motors and other electronic systems and devices on an aircraft may be provided by an aircraft power system.
An example of a type of electric motor used on aircraft, and for many other applications, is a brushless DC electric motor. These types of motors are also known as electronically commutated motors. Brushless DC motors are powered by direct current (DC) electricity and have electronic commutators.
A brushless DC motor has permanent magnets which rotate and a fixed armature which comprises the stator windings. An electronic controller continually switches the phase of power provided to the windings to keep the motor turning. The controller may employ a solid stat circuit to provide timed power distribution to the motor windings.
Because the controller must direct the rotor rotation, the controller requires some means of determining the rotor's orientation relative to the stator windings. Some brushless DC motors use Hall effect sensors or a rotary encoder to directly measure the rotor's position. Others measure the back electromotive force (EMF) in the undriven windings to infer the rotor position. Controllers of this latter type are often called sensorless controllers. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position.
The controller for a brushless DC motor may provide three bi-directional outputs to control the driving of DC power to the motor windings. The outputs may be controlled by a logic circuit. Simple controllers may employ comparators to determine when the output phase should be advanced. More advanced controllers may employ a microcontroller to manage acceleration, control speed, and fine tune-motor efficiency. Motor controllers of this type may be referred to as electronic speed controllers.
The controller may control the power that is provided to the windings of a DC motor by controlling the switches in an H-bridge. The H-bridge couples the DC power source to the windings of the DC motor. For example, a three-phase H-bridge has six switches for coupling the DC power source to three motor windings of a DC motor. The switches of the H-bridge may be controlled by the controller to drive a current in either direction on each of the motor windings. For example, without limitation, the H-bridge may be implemented using solid state switching devices such as metal-oxide-semiconductor field-effect transistors (MOSFETs).
Various methods may be used to control the switches in an H-bridge to modulate the current in the windings of a brushless DC motor. However, existing methods for modulating the current in the motor windings of a brushless DC motor have drawbacks. A method for controlling the current in the windings of a brushless DC motor that overcomes these drawbacks is desirable.
Accordingly, it would be desirable to have a method and apparatus that takes into account one or more of the issues discussed above as well as possibly other issues.